[Haskell-cafe] Re: Parallel Pi

Ketil Malde ketil at malde.org
Fri Mar 19 05:00:36 EDT 2010

Daniel Fischer <daniel.is.fischer at web.de> writes:

> 3.06GHz Pentium 4, 2 cores.

[I.e. a single-core hyperthreaded CPU]

> I have mixed results with parallelism, some programmes get a speed-up of 
> nearly a factor 2 (wall-clock time), others 1.4, 1.5 or so, yet others take 
> about the same wall-clock time as the single threaded programme, some - 
> like this - take longer despite using both cores intensively.

Given the negative press around HT, I'm surprised you see this good
results on many programs.  I thought the main benefit from Intel's HT was
to reduce the impact of memory latency, that is, when one thread was
blocking on memory, it could switch immediately to anther, ready-to-run,
thread. (I may be misunderstanding this, though).

I think the general consensus was a 10-15% speedup from HT.

Anyway, the thing to get these days is of course Nehalem, A.K.A. Core
i{3,5,7}, which seems to give a nice speedup over Core 2.  Among other
things, it dynamically overclocks the busy cores (using the
more market-friendly term "turbo mode"), making it even harder to
compare performance reliably.  Interesting times.

If I haven't seen further, it is by standing in the footprints of giants

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